1. IN A ROTATION ROCK DRILL INCLUDING A HOUSING HAVING THEREIN A RECIPROCATING PISTON, A RATCHET RING SURROUNDING THE PISTON AND HAVING SPIRAL SPLINED ENGAGEMENT THEREWITH, A PAIR OF SPRING LOADED PAWLS PIVOTABLE UNDER THE CONSTANT LOAD OF THE SPRINGS INTO ENGAGEMENT WITH THE RATCHET RING AND PIVOTABLE AGAINST THE LOAD OF THE SPRINGS OUT OF SUCH ENGAGEMENT; SLEEVE CAM MEANS SURROUNDING THE RATCHET RING HAVING ROTATION RELATIVE TO THE RATCHET RING AND PAWLS FOR CAMMING THE PAWLS OUT OF ENGAGEMENT FROM THE RATCHET RING, WHEREIN A SHIFTER LEVER MANUALLY SLIDABLE IN THE HOUSING TRANSVERSELY OF THE PERIPHERAL SURFACE OF THE SLEEVE CAM MEANS HAS A KEY ENGAGEMENT WITH THE LATTER FOR CAUSING ROTATION OF THE SLEEVE ACCORDINGLY AS THE SHIFTER LEVER IS SELECTIVELY MANUALLY SLID IN ONE DIRECTION OR THE OTHER, AND SPRING LOADED DETENT STOP MEANS IS PROVIDED IN THE HOUSING FOR RELEASABLY LIMITING THE EXTENT OF MOVEMENT OF THE SHIFTER LEVER IN EITHER DIRECTION TO A PREDETERMINED DEGREE.